1. Field of the Invention
This invention relates to improved structures for light valves and the methods of manufacture necessitated by these improved structures.
2. Brief Description of the Prior Art
Previously, most light valves have been based on mechanical and/or electrical operation. For example, the means for varying the amount of sunlight admitted into buildings includes curtains, venetian blinds, seasonal paint for greenhouses, and skylids. Skylids are automatic mechanical shutters which are actuated thermally by sunlight. Mechanical light valves for buildings are surveyed in "Thermal Shutters and Shades" by W. A. Shurcliff, Brick House, Andover, Mass.
Other examples of light valves are thin layers of liquid crystals in wrist watches and other displays, electrochromic coatings for automobile rear view mirrors which turn dark electronically, Kerr electro-optic cells for laser modulation, and photochromic glass for eyeglasses. Other light valves are discussed in "Electrochromic and Thermochromic Materials for Solar Energy Applications" USDOE Report LBL-18299.
Mechanical and electrical light valves suffer from cost and reliability problems. Mechanical and electrical light valves require both a separate sensor to determine which state of the light valve is desired, and a means for the sensor to activate the light valve.
For many applications, light valves should not become highly absorptive of light when they are in their non-transmissive state. For windows and skylights, for example, it is preferred that incident sunlight is reflected rather than absorbed so that it does not become a heat load for the building. Light absorption is a problem with the above mentioned photochromic glass, and photochromic, electrochromic, and thermochromic coatings, which all turn dark.
White paint for greenhouses is labor intensive and thus expensive. Since it is applied and removed seasonally, plant growth is slower than it would be with a light valve whose transmission varied over a time scale of minutes.
A light valve which turns from transmissive to reflective upon heating above its transition temperature and then turns transmissive again upon cooling is called here a "thermo-optical shutter." Since it is activated automatically by its local temperature, a thermo-optical shutter may be used without any external control mechanism in some applications, such as solar heating. For some other applications, such as skylights, it is preferred that the thermo-optical shutter be activated by ambient light intensity instead of local temperature.
Structures, preparation methods, and materials for thermo-optical shutters which are related to the present invention, and which are patented by the inventor, are given in U.S. Pat. Nos. 3,953,110 and 4,085,999 and 4,307,942.
Previous thermo-optical shutters related to the present invention have overcome the problems listed above for other types of light valves, but they have not performed as well as the present invention because: they do not retain their opacity for as long; or lack shear strength; or are difficult to manufacture; or are prone to puncture damage; or are prone to freezing damage; or are activated only by their local temperature, and not by local light intensity; or do not have a transition temperature which can be varied continuously over a range during the manufacture of the shutter.